1. Field of the Invention
The present invention relates to an image display device for displaying an image, and more particularly to an image display device that displays an image by actively driving a multiplicity of two-dimensionally arranged organic EL (Electro-Luminescent) elements.
2. Description of the Related Art
EL displays for displaying a dot matrix image in which a multiplicity of organic EL elements are two-dimensionally arranged have currently been developed as image display devices for displaying various images in locations subject to radical changes in illumination, such as the interior of an automobile. Organic EL elements are light-emitting elements that spontaneously emit light and can be driven by a low-voltage direct current.
Methods of driving organic EL elements include passive matrix drive methods and active matrix drive methods. An active matrix drive method can achieve high luminance with high efficiency because the organic EL elements are lit continuously until updating of the display image.
As an example of an image display device of the prior art, explanation is presented with reference to FIG. 1 and FIG. 2 regarding an EL display that actively drives organic EL elements.
As shown in FIG. 1, EL display 1 that is presented as an example of the prior art includes organic EL element 2 as well as power supply line 3 and ground line 4 as a pair of power supply electrodes. A predetermined drive voltage is constantly applied to power supply line 3, and ground line 4 is constantly maintained at 0 V, which is the reference voltage.
Organic EL element 2 is directly connected to ground line 4 but is connected to power supply line 3 by way of drive TFT (Thin-Film Transistor) 5. This drive TFT 5 includes a gate electrode, and the drive voltage that is applied to ground line 4 from power supply line 3 is supplied to organic EL element 2 according to a data voltage that is applied to this gate electrode.
One end of capacitor 6 is connected to the gate electrode of drive TFT 5, and the other end of this capacitor 6 is connected to ground line 4.
Data line 8 is connected to this capacitor 6 and the gate electrode of drive TFT 5 by way of switching TFT 7, which is a switching element, and scan line 9 is connected to the gate electrode of this switching TFT 7.
A data voltage for driving the light emission intensity of organic EL element 2 is supplied to data line 8, and a scan voltage for controlling switching TFT 7 is applied to scan line 9. Capacitor 6 holds the data voltage and applies it to the gate electrode of drive TFT 5, and switching TFT 7 turns the connection between capacitor 6 and data line 8 ON and OFF.
In EL display 1, (Mxc3x97N, M and N are predetermined natural numbers) organic EL elements 2 are arranged two-dimensionally in M rows and N columns (not shown in the figures), and M rows of data lines 8 and N columns of scan lines 9 are connected in a matrix to these M rows and N columns of organic EL elements 2. In the figures, the term xe2x80x9crowxe2x80x9d refers to the dimension parallel to the vertical direction and the term xe2x80x9ccolumnxe2x80x9d refers to the dimension parallel to the horizontal direction, but this is merely a matter of definition, and the reverse case is also possible.
EL display 1 according to the above-described construction is capable of driving organic EL elements 2 with variable light emission intensity. In such a case, a scan voltage is applied to scan line 9 and switching TFT 7 is controlled to an ON state as shown in FIG. 2b and FIG. 2c, and a data voltage from the data line that corresponds to the light emission intensity of organic EL element 2 in this state is supplied to and held in capacitor 6 as shown in FIG. 2e. 
The data voltage held by this capacitor 6 is applied to the gate electrode of drive TFT 5 as shown in FIG. 2d, and as a result, as shown in FIG. 2f, the drive voltage that is constantly generated at power supply line 3 and ground line 4 is supplied to organic EL element 2 by drive TFT 5 in accordance with the gate voltage. As a result, organic EL element 2 emits light at an intensity that accords with the data voltage that was supplied to data line 8.
In EL display 1, data voltage and scan voltage are applied in a matrix to M rows of data lines 8 and N columns of scan lines 9, and each of M rows and N columns of organic EL elements 2 are therefore lit at different intensities, thereby displaying a dot-matrix image with the gray scale expressed in pixel units.
In such a case, the scan voltage is applied in order one column at a time to N columns of scan lines 9 in EL display 1 as shown in FIG. 2a and FIG. 2b, and when this scan voltage is being applied, one column of M data voltages is therefore applied in order to M rows of data lines 8.
The state in which the drive voltage is applied to organic EL element 2 in accordance with the data voltage that is held by capacitor 6 as described in the foregoing explanation continues even when switching TFT 7 is placed in the OFF state by the scan voltage of scan line 9. Organic EL element 2 thus continues emission that is controlled to a predetermined luminance until the next instance of control, and EL display 1 therefore is capable of displaying a bright and high-contrast image.
In EL display 1 in which organic EL elements 2 are actively driven as described above, however, organic EL elements 2 have a short life. Various explanations can be offered, but characteristically, it is clear that continuous application of the drive voltage of the same polarity to organic EL electrodes 2 results in a short life of the elements.
In an EL display (not shown) that passively drives organic EL elements 2, for example, it has been confirmed that organic EL elements 2 have a longer life than in the case of active drive because the polarity of voltage applied to organic EL elements 2 reverses during the drive process. A passive-type EL display as described hereinabove, however, is incapable of driving organic EL elements 2 at both high luminance and high contrast, and such a display is therefore difficult to use in devices requiring high luminance.
It is an object of the present invention to provide an image display device capable of employing active drive to light organic EL elements at high luminance and high efficiency while enabling longer life of the elements.
According to one aspect of the present invention, (Mxc3x97N) organic EL elements are arranged two-dimensionally in M rows and N columns, (Mxc3x97N) data voltages that individually set the light-emission luminance of these (Mxc3x97N) organic EL elements are applied in order N times for each of the M rows of data lines, and the scan voltage is applied in order to the N columns of scan lines in synchronization with the data voltages that are applied to these M rows of data lines. The scan voltage that is applied in order to these N columns of scan lines causes the M rows and N columns of switching elements to turn on one column at a time, and the (Mxc3x97N) data voltages that are applied from the M rows of data lines in accordance with the ON state of these M rows and N columns of switching elements are individually held by M rows and N columns of data voltage holding means. The drive voltage that is constantly applied to the power supply electrode is applied to the (Mxc3x97N) organic EL elements by the M rows and N columns of drive transistors in individual correspondence to the held voltage of the (Mxc3x97N) voltage holding means. The M rows and N columns of organic EL elements are thus actively driven at individually differing luminances to display a multiple gray-scale dot matrix image.
Immediately before the application of the scan voltage to the scan line of the nth column, however, a conduction control element halts the application of the drive voltage to the M organic EL elements of the nth column. As a result, conduction to the actively driven organic EL elements is halted an instant before performing display control of the image, even when an image is continuously displayed at the same luminance, thereby enabling a longer life of the organic EL elements.
According to another aspect of the present invention, a conduction control element applies a reverse voltage, which has the opposite polarity of the drive voltage, to the M organic EL elements of the nth column immediately before the scan voltage is applied to the scan line of the nth column. As a result, the polarity of voltage that is applied to actively driven organic EL elements is reversed an instant before performing display control of the image, even when an image is continuously displayed at the same luminance, thereby enabling a longer life of organic EL elements.
In an embodiment, when a scan voltage is applied to the scan line of the (nxe2x88x92a)th column, a conduction control element halts the application of the drive voltage to the organic EL elements of the nth column. As a result, the application of the drive voltage to the M organic EL elements of the nth column can be simply and reliably halted at a desired timing immediately before the scan voltage is applied to the scan line of the nth column.
In an embodiment, when the scan voltage is applied to the scan lines of the (nxe2x88x92a)th column, a conduction control element applies a reverse voltage to the organic EL elements of the nth column. As a result, application of a reverse voltage, which has the opposite polarity of the drive voltage, to the M organic EL elements of the nth column can be simply and reliably performed at a desired timing immediately before the scan voltage is applied to the scan lines of the nth column.
In an embodiment, when the scan voltage is applied to the scan lines of the (nxe2x88x92a)th column, a conduction control element halts the application of the drive voltage to the organic EL elements of the nth column and applies a reverse voltage. As a result, the application of a reverse voltage, which has a polarity opposite that of the drive voltage, to the M organic EL elements of the nth column can be simply and reliably carried out at a desired timing immediately before the scan voltage is applied to the scan lines of the nth column.
In an embodiment, when a scan voltage is applied to the scan lines of the (nxe2x88x92b)th column, a conduction control element halts the application of the drive voltage to the organic EL elements of the nth column, and when a scan voltage is applied to the scan lines of the (nxe2x88x92a)th column, the conduction control element applies a reverse voltage to the organic EL elements of the nth column. Accordingly, a reverse voltage can be reliably conducted to the organic EL elements after the application of the drive voltage to the organic EL elements has been reliably halted.
In an embodiment, when a scan voltage is applied to the scan lines of the (nxe2x88x92a)th column, a conduction control element discharges the voltage held by a voltage holding means of the nth column. As a result, application of the drive voltage to the organic EL elements can be simply and reliably halted by controlling the voltage holding means.
In an embodiment, when a scan voltage is applied to the scan lines of the (nxe2x88x92a)th column, a conduction control element disconnects the connection between the power supply electrode and the organic EL elements of the nth column. As a result, the application of drive voltage to the organic EL elements can be reliably halted.
In an embodiment, a conduction control element conducts the scan voltage that is applied to the scan lines of the (nxe2x88x92a)th column to the organic EL elements of the nth column as the reverse voltage. As a result, the scan voltage can be used as the reverse voltage that is conducted to the organic EL elements, and a proper reverse voltage can be reliably generated by means of a simple construction.
In an embodiment, when a scan voltage is applied to the scan lines of the (nxe2x88x92b)th column, a conduction control element discharges the voltage that is held by the voltage holding means of the nth column and conducts the scan voltage that is applied to the scan lines of the (nxe2x88x92a)th column to the organic EL elements of the nth column as the reverse voltage. Accordingly, the application of drive voltage to the organic EL elements by the scan voltage of the scan lines of the (nxe2x88x92b)th column can be halted through control of the voltage holding means, the scan voltage of the scan lines of the (nxe2x88x92a)th column can be conducted as the reverse voltage to the organic EL elements for which this current conduction has been halted, and a reverse voltage can be applied to organic EL elements for which the drive voltage has been completely halted.
In an embodiment, when a scan voltage is applied to the scan lines of the (nxe2x88x92b)th column, a conduction control element disconnects the connection between the power supply electrode and the organic EL elements of the nth column and conducts the scan voltage that is applied to the scan lines of the (nxe2x88x92a)th column to the organic EL elements of the nth column as a reverse voltage. Accordingly, the application of drive voltage to the organic EL elements by the scan voltage of the scan lines of the (nxe2x88x92b)th column can be halted by disconnecting the power supply electrodes, the scan voltage of the scan lines of the (nxe2x88x92a)th column can be conducted as the reverse voltage to the organic EL elements for which this current conduction has been halted, and a reverse voltage can be applied to the organic EL elements for which the drive voltage has been completely halted.
In an embodiment, a is equal to 1. Accordingly, the conduction control element controls conduction to organic EL elements when the scan voltage is applied to the scan lines of the preceding column, but control of conduction to the organic EL elements of the first column is effected when the scan voltage is applied to the scan lines of the Nth column, which is the last column. Accordingly, the control of conduction to the organic EL elements of the first column at a proper timing and by a simple construction can be realized by a construction in which a conduction control element controls conduction to organic EL elements when the scan voltage is applied to the scan lines of the preceding column.
In an embodiment, a is equal to 1. Accordingly, a conduction control element controls conduction to organic EL elements when the scan voltage is applied to the scan lines of the preceding column, but a dummy scan voltage is applied to a dummy line that is provided parallel to the scan line of the first column immediately before application of the first-column scan voltage. Accordingly, control of conduction to the organic EL elements of the first column is performed when the dummy scan voltage is applied to the dummy line. As a result, the control of conduction to the organic EL elements of the first column at a proper timing and by a simple construction can be realized by a construction in which the conduction control element controls conduction to organic EL elements when the scan voltage is applied to the preceding scan line.
In an embodiment, a is equal to 1 and b is equal to 2. Accordingly, a conduction control element halts the drive voltage that is applied to organic EL elements when the scan voltage is applied to the scan line of the second preceding column, and the conduction control element applies a reverse voltage to organic EL elements when the scan voltage is applied to the scan lines of the preceding column. However, the drive voltage to the organic EL elements of the first column is halted when the scan voltage is applied to the scan line of the (Nxe2x88x921)th column, and a reverse voltage is conducted to the organic EL elements of the first column when the scan voltage is applied to scan line of the Nth column. The drive voltage to the organic EL elements of the second column is halted when the scan voltage is applied to the scan lines of the Nth column. Accordingly, conduction to the organic EL elements of the first column and second column can be controlled at a proper timing and by a simple construction by a construction in which the conduction control element halts the drive voltage that is applied to the organic EL elements when the scan voltage is applied to the second preceding scan line and applies a reverse voltage to organic EL elements when the scan voltage is applied to the scan line of the preceding column.
In an embodiment, a is equal to 1 and b is equal to 2. Accordingly, a conduction control element halts the drive voltage that is applied to organic EL elements when the scan voltage is applied to the scan line of the second preceding column, and the conduction control element applies a reverse voltage to organic EL elements when the scan voltage is applied to the scan lines of the preceding column. However, first and second dummy scan voltages are applied to first and second dummy lines that are provided parallel to the scan line of the first column immediately before application of the first-column scan voltage. As a result, the drive voltage to the organic EL elements of the first column is halted when the scan voltage is applied to the first dummy line, and a reverse voltage is conducted when the scan voltage is applied to the second dummy line. The drive voltage to the organic EL elements of the second column is halted when the scan voltage is applied to the second dummy line. Accordingly, conduction to the organic EL elements of a first column and second column at a proper timing and by a simple construction can be realized by a construction in which a conduction control element halts the drive voltage that is applied to organic EL elements when the scan voltage is applied to the scan line of the second preceding column and applies a reverse voltage to organic EL elements when the scan voltage is applied to the scan line of the preceding column.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.